In the prior art, such a type of the shock absorber is interposed between a chassis and an axle of a vehicle to suppress vibration. The shock absorber of the prior art includes, for example, a cylinder, a piston slidably inserted into the cylinder to partition the inside of the cylinder into an expansion-side chamber of the piston rod side and a contraction-side chamber of the piston side, a first flow passage that causes the expansion-side chamber provided in the piston and the contraction-side chamber to communicate with each other to generate a damping force, a second flow passage opened from the leading edge of the piston rod to the side portion to cause the expansion-side chamber and the contraction-side chamber to communicate with each other, a housing provided with a compression chamber connected to the middle of the second flow passage and provided in the leading edge of the piston rod, a free piston slidably inserted into the compression chamber to partition the compression chamber into an expansion-side compression chamber and a contraction-side compression chamber, and a coil spring that biases the free piston. That is, the expansion-side compression chamber communicates with the expansion-side chamber through the second flow passage, and the contraction-side compression chamber communicates with the contraction-side chamber through the second flow passage, likewise (for example, see JP 2008-215459 A).
In the shock absorber having such a structure, the compression chamber is partitioned by the free piston into the expansion-side compression chamber and the contraction-side compression chamber, and the expansion-side chamber and contraction-side chamber do not directly communicate with each other through the second flow passage. However, as the free piston moves, a volume ratio between the expansion-side compression chamber and the contraction-side compression chamber changes. Therefore, a liquid inside the compression chamber accesses the expansion-side chamber and the contraction-side chamber depending on a shift amount of the free piston. For this reason, apparently, the expansion-side chamber and the contraction-side chamber communicate with each other through the second flow passage. In addition, in such a type of the shock absorber, a ratio of the flow rate of the second flow passage against the flow rate of the first flow passage is insignificant for a low-frequency vibration input. Meanwhile, for a high-frequency vibration input, a ratio of the flow rate of the second flow passage against the flow rate of the first flow passage increases.
Therefore, such a type of the shock absorber can generate a strong damping force for a low-frequency vibration input and generate a weak damping force for a high-frequency vibration input by virtue of a damping force attenuation effect. As a result, it is possible to reliably generate a strong damping force when the input vibration frequency is low, for example, when a vehicle turns. In addition, when the input vibration frequency is high, for example, when a vehicle travels on an uneven road surface, it is possible to generate a weak damping force to improve a vehicle ride quality.